The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for driving bipolar junction transistors. Merely by way of example, the invention has been applied to a power converter. But it would be recognized that the invention has a much broader range of applicability.
Power converters are widely used for consumer electronics such as portable devices. The power converters can convert electric power from one form to another form. As an example, the electric power is transformed from alternate current (AC) to direct current (DC), from DC to AC, from AC to AC, or from DC to DC. Additionally, the power converters can convert the electric power from one voltage level to another voltage level.
The power converters include linear converters and switch-mode converters. The switch-mode converters often have higher efficiency than the linear converters, and usually use power transistors as switches. For example, the power transistors are field effect transistors, bipolar junction transistors, insulated gate bipolar transistors, and/or other types of transistor. The power switches often receive control signals from pulse-width-modulated (PWM) controllers and/or pulse-frequency-modulated (PFM) controllers. The control signals can be adjusted in response to output loads by sensing output voltages or currents.
FIG. 1 is a simplified conventional switch-mode converter with power switch. A switch-mode power converter 100 includes an over-current-protection (OCP) comparator 110, a PWM controller component 120, a gate driver 130, a power switch 140, a primary winding 150, and a secondary winding 160. The power switch 140 is a field effect transistor such as a high-voltage power MOSFET and is used to control power delivered to the secondary side of the switch-mode power converter 100. For example, if the current of the primary winding 160 is greater than a limiting level, the PWM controller component 120 turns off the power switch 140 and shuts down the switch-mode power converter 100.
As a power switch, a high-voltage power MOSFET and a high-voltage power bipolar NPN transistor can achieve similar performance for low power applications. But the high-voltage power MOSFET is often significantly more expensive than the high-voltage power bipolar NPN transistor. Hence it is often desirable to use the bipolar NPN transistor instead of the MOSFET in a switch-mode power converter for low power applications to lower the costs.
But the operation of a MOSFET and the operation of a bipolar junction transistor (BJT) are different. The MOSFET is a voltage controlled device; in contrast the BJT is a current controlled device. As an example, for MOSFET, the drain-to-source current is turned on and off by the gate voltage. As another example, for BJT, the collector current is turned on and off by the base current. Therefore, a system and method for driving a power MOSFET often cannot be used for driving a power BJT.
Hence it is highly desirable to improve techniques for driving bipolar junction transistors.